The respiratory act among animals takes place with the knowledge and with the assist ance and implied will of the individual. Animals are informed of the necessity of re spiring by the feeling of a want, an uneasiness, just as they are admonished of the necessity of taking aliment by the painful sensations denominated hunger and thirst.
The essence of respiration in the two grand classes of organized beings would therefore appear to be different, and might be made the ground of a definitive distinction between the members of each kingdom. Carbon is the object for which the respiration of plants is instituted ; oxygen the end for which re lations are established between animals and the atmosphere. Another grand difference be tween the respiration of plants and animals is the involuntariness of the act in the one, and its voluntariness in the other, its occurrence with unconsciousness in the one, and with con sciousness in the other.
The nutrient juices thus prepared have now to be distributed ; this is done by means of a peculiar motion impressed upon the fluids m virtue of a vital law with the nature of which we are still very imperfectly acquainted. Let us use the word circulation in a sense implying motion generally, not motion in a circle to designate the act by which in the organized world the nutritive juices are dis tributed through the frames of the objects composing it.
can be no doubt of the existence of a circulation among vegetables ; in many species currents in opposite directions have even been seen with the aid of the micro scope, and this not only among the lowest and most simple in their structure of the class, but also in the highest and most complicated. The circulation of vegetables appears to take place within two different congeries of vessels, ex tremely numerous, and disposed according to their nature in different parts of the plant. The vessels that pump or transmit the sap from the roots to the leaves, for instance, as we have already had occasion to state, run within the woody parts of plants • those that receive the modified juices of the leaves, again, take their course downwards within the bark. These two sets of vessels anastomose within the substance of the leaves, but no where else; the second set can alone be said to have a distribution throughout the vegetable, for every part appears to depend on them for its supply of nourish ment, even the extreme points of the roots, which were themselves the first instruments in collecting the aliment still unfit for the purposes of nutrition. The best informed vegetable physiologists are of opinion that the nutritive fluid once sent off from the leaves never finds its way back to these organs again ; it is ab sorbed or fixed by the different parts or struc tures to which it is distributed, ministering to their increment generally, and enabling each to manifest its specific function in the vegetable economy.
In this motion of the fluids of vegetables it is evident that there is little analogous to what we find within the bodies of animals somewhat elevated in the scale. But let us first cast a hasty glance at what does take place within this other division of the organic kingdom be fore instituting a comparison between the func tions of circulation in the two. All animals, from the mammalia downwards to the entozoa, —birds, reptiles, fishes, the mollusca, crustacea, arachnjda, insecta, and, among the radiata, the holothuria, echini, and asteria, include within their organisms particular canals or vessels for containing and distributing their nutrient juices, and within which, moreover, these are in motion in a circle. In the acalepha we still find canals branching off from the digestive cavity and dis tributing the nourishment there prepared to the different parts of the body : in these, however, we no longer.find any contrivance for establish ing a circular motion in the nourishing juices. Still lower in the scale, among the polypes and actinew, for example, we discover no branched appendages or canals for the distribution of the nutrient fluids; those prepared in the stomachs of the animals appear to penetrate their sub stance directly, and to permeate the homo geneous cellular tissue of which they consist.
In the tribes which have a circulation, in the strict sense of that word, we find two or ders of vessels,—arteries and veins, in which the nutritive juices, or blood, moves respec tively in opposite directions, from the trunks towards the branches in the one, from the branches towards the trunks in the other. These vessels anastomose freely by their ex tremities, which terminate and originate in every part of the body, and, farther, meet in a common central cavity, which, when furnished with muscular parietes, is entitled heart. With in the circle of vessels thus established, the nutrient fluid of animals is in perpetual or next to perpetual motion during the term of their lives. In the higher classes the main agent in producing this motion is the central organ in which the veins and arteries meet; but it is not the only cause of the circulation, this act going on vigorously in circles and in situations wherein the heart's action can have very little influence, and in some tribes where the heart is even altogether wanting.